The Stomach Flashcards
What are the main GI tract functions applicable to the stomach?
It is mainly involved in mechanical processing, digestion and secretion
When may ingestion be a function of the stomach?
If the stomach is used directly as an input mechanism through percutaneous endoscopic gastrostomy
What is percutaneous endoscopic gastrostomy?
A tube passes through the anterior abdominal wall into the stomach
It is anchored by a balloon and is used for feeding directly
When may the stomach be a route of excretion or absorption?
It is only a route of excretion through vomiting
Lipid soluble substances can occasionally be absorbed through the wall of the stomach
Where does the stomach begin and terminate?
The oesophagus almost immediately enters the stomach after passing through the diaphragm
It terminates at the duodenum
What is the interior surface of the stomach comprised of?
A mucosal layer of epithelium
What are rugae?
Folds of epithelium in the stomach
What is the purpose of rugae?
As the stomach fills with food, the epithelium can flatten out
There will not be a rise in tension as the stomach fills, allowing it to have a wide range of volumes
What is the muscular layer of the stomach?
This is the layer outside of the epithelium
It consists of longitudinal, circular and oblique muscle layers
What does contraction of the oblique muscle layer help with?
It helps to mix and liquify the food
What is the lesser omentum?
Where does it start and terminate?
It is a mesentery formed on the lesser curvature of the stomach
It runs from the lesser curvature to the porta hepatis (entrance to the liver)
What is the greater omentum?
How many sheets of peritoneum is it formed from?
It is a double sheet of peritoneum that descends from the greater curvature
It then picks up another double sheet of mesentery from the colon to form the greater omentum
It consists of 4 layers of peritoneum
In which region of the stomach is the muscle layer much thicker and stronger?
Why?
In the pyloric-antral region
This allows food to be squeezed back and forth within this region until it is small enough to pass through the pyloric sphincter
What is the main function of the upper part of the stomach?
It does have peristaltic waves, but its main function is to be able to relax
What is the luminal secretion of the LES and cardia?
What are the functions of this region?
Mucus and HCO3-
The main functions are:
- prevention of reflux
- entry of food
- regulation of belching
What is the luminal secretion of the fundus and body?
- H+
- intrinsic factor
- mucus
- HCO3-
- pepsinogens
- lipase
What are the main functions of the fundus and body?
- acts as a reservoir
2. provides tonic force during emptying
What is the luminal secretion of the antrum and pylorus?
What are the main functions of this region?
Mucus and HCO3-
This region is involved in:
- mixing, grinding and sieving of food
- regulation of gastric emptying
What is the lower oesophageal sphincter (LES)?
What is its function?
It is at the lower part of the oesophagus
It constricts to control flow into and out of the stomach
It helps to prevent acid refluxing back up into the stomach
Why is the stomach acidic even though the fundus and body secrete bicarbonate?
Both acid and bicarbonate are secreted
The mucus traps the bicarbonate close to the surface, whilst the acid is released into the lumen
When will the pyloric sphincter allow contents of the stomach to pass through into the duodenum?
Once the foodstuffs have been ground down to small enough molecules
How does the pyloric sphincter regulate emptying of the stomach?
Why is this important?
It controls the rate at which substances enter into the duodenum
This gives time for the small bowel to process the contents
What are the 4 main regions of the stomach?
- cardia
- fundus
- body
- pylorus
What are the different parts within the pylorus?
The pyloric antrum is the wider end of the pylorus which connects to the body
The narrower end is the pyloric canal, which connects to the duodenum
What are the fundus and body of the stomach responsible for?
- accepting and holding food
- relaxation
- accommodation
What is the antrum and pylorus responsible for?
The antrum is responsible for churning and grinding food
The antrum and the pylorus are responsible for the emptying of the stomach
What is accommodation?
What does it require?
It is an active relaxation process that is neurally controlled
It requires a vago-vagal reflex
What does accommodation depend on?
Stretch sensors in the stomach which signal via the vagus nerve
What happens when descending signals via the vagus nerve act on the enteric nervous system?
It results in relaxation and dilatation of the fundus and body of the stomach
This allows up to 1 litre of food to be stored in the stomach with very little rise in pressure
What is the motion of the fundus and body like during digestion?
They are relatively still
Occasionally peristaltic waves will squeeze off another “bite” and pass it to the antral pump
How can the antral pump adapt to the type of food in the stomach?
Liquid is allowed to pass through the pyloric sphincter
Thicker material is churned and ground until it is smooth enough to be passed into the duodenum
What happens in vagotomy (vagus nerve cut) as the stomach begins to fill?
The brain has no way of knowing the stomach is filling so the pressure progressively rises
There is no accommodation
How does the amount of fluid the stomach can hold vary in people with and without a vagotomy?
In normal people, much more fluid can enter the stomach without the pressure rising
This is because the stomach expands as it fills due to accommodation via the stretch receptors
What size must particles be to be unable to pass through the pyloric sphincter?
Larger than 1 - 2 mm
What does the duodenum sense and what hormones will it secrete in response?
It senses the delivery of acids, amino acids and lipids
It releases CCK, secretin and GIP
This decreases gastric motility and emptying
How is the nervous system involved in controlling gastric emptying?
The vagus nerve innervates the duodenum and helps regulate gastric emptying
The enteric nervous system also controls this, and includes feedback from the duodenum
How does the rate of gastric emptying vary after eating a saline and acidic meal?
The stomach empties much faster after a saline meal
If an acidic meal is consumed, the acid signals as it reaches the duodenum
This slows gastric emptying to allow more time for processing
What types of meals take the longest to process?
Oleate (oil) meals
The duodenum senses oily foods and slows down gastric emptying to give it longer to process the food
Where are the gastric glands found?
They are large and buried deep in the stomach
what are the surface epithelial cells that continue in to the neck of the gastric gland?
Mucous cells
They secrete mucous which helps to protect the cells from acid and enzymes
What else will the surface epithelial cells entering into the gastric gland secrete?
Bicarbonate
This sits in the mucous layer and stops the acidic pH from damaging the surface of cells
What are gastric pits and glands mostly classified as?
What does this mean?
They are mainly oxyntic
These are epithelial cells which secrete hydrochloric acid and intrinsic factor
Where are oxyntic cells found?
In the gastric glands found in the lining of the fundus and in the cardia of the stomach
(also called parietal cells)
What is the role of G cells ?
Where are they mostly found?
They secrete gastrin into the bloodstream
They are mainly found in the antral/pyloric glands
Why are G cells referred to as ‘signalling cells’?
They sense what is occurring in the stomach and can release gastrin accordingly
They signal to other parts of the stomach, the duodenum and pancreas
What are Chief cells and what is their role?
They are also called peptic cells
They secrete pepsinogen into the stomach
What are the 2 forms of gastrin?
What is similar about them?
G17 (17 amino acids)
G34 (34 amino acids)
The C-terminal 17 amino acids of G34 are the same as G17
What is the difference between G17 and G34?
G17 is the main form of gastrin secreted from the antrum
G34 is the main form of gastrin secreted from the duodenum
What is the main gastrin receptor?
What is meant by gastrin being homologous with CCK-PZ?
The main gastrin receptor is CCK-B
Gastrin is homologous with CCK-PZ as the C-terminal 5 amino acids are the same
What 2 things can stimulate release of gastrin?
- G cells detecting the presence of amino acids and proteins in the lumen
- parasympathetic input
What is the parasympathetic input like which stimulates gastrin release?
It is cephalic and in response to gastric stretch
Vagal stimulants being to initiate gastrin release as part of the cephalic phase - before food is actually eaten
What is gastrin release inhibited by?
What type of mechanism is this?
Luminal H+
This is a negative feedback mechanism
As acid begins to accumulate in the lumen, it inhibits gastrin production to prevent more acid being released
What happens to the stomach if there is no gastrin present?
The stomach eventually atrophies
What are the main actions of gastrin on the stomach?
- stimulate acid secretion
2. promote mucosal growth
What is somatostatin?
a peptide hormone which exists in 14 and 28 amino acid forms
Where is somatostatin released from?
By which mechanisms does it work?
the D cells of the stomach, duodenum and pancreas
It mainly acts in a paracrine fashion (but can also be endocrine)
What will stimulate and inhibit the release of somatostatin?
Release is stimulated by luminal H+
The release is inhibited by acetylcholine (parasympathetic activity)
What is the function of somatostatin?
It acts on G cells to inhibit the release of gastrin
It also inhibits the release of CCK and secretin in the duodenum
What do parietal (oxyntic) cells store in their cytoplasm?
They store many tubulovesicles within their cytoplasm
The proton pumps which secrete acid sit in the vesicles
Why is there no acid secretion at rest, referencing the oxyntic cells?
The tubulovesicles in the oxyntic cells are not embedded in the membrane
What happens to the tubulovesicles when gastrin and acetylcholine stimulate acid secretion?
The vesicles fuse into the indentations in the cell membrane to form channels called canaliculi
The proton pumps now sitting in the membrane can secrete acid
What 2 hormones stimulate gastric acid secretion?
What hormone inhibits gastric acid secretion?
gastrin and acetylcholine
inhibited by somatostatin
By which pathways do gastrin, ACh and somatostatin act?
How are they mediated?
They work by both direct and indirect pathways
The indirect effect is mediated by histamine
What is the active pump involved in acid secretion and how does it work?
The H+/K+ ATPase
It ejects protons from the cell and picks up K+ ions
What is the source of protons for the H+/K+ ATPase?
Carbonic anhydrase converts carbon dioxide and water into bicarbonate ions and H+
The bicarbonate ions are pushed out into the circulation
The H+ are pushed into the lumen of the stomach by the H+/K+ ATPase
What is meant by the “alkaline tide”?
What is it coupled with and what does it result in?
The movement of bicarbonate ions into the circulation whenever substances are eaten
It couples movement of H+ ions into the stomach lumen
It causes the blood pH to rise slightly
What drives the process behind the action of the H+/K+ ATPase?
Interactions with the gastrin receptor (GPCR) and activity from the parasympathetic nervous system
They drive the process through a cascade which leads to activation of the H+/K+ ATPase
What type of cells mediate the indirect effect of gastrin, ACh and somatostatin?
enterochromaffin-like cells (ECL cells)
they sit within the wall of the stomach but have a nerve cell-like function
What do enterochromaffin-like cells detect?
What is the outcome?
They detect the presence of gastrin and the activity of the parasympathetic nervous system
They release histamine
Histamine acts through a histamine receptor to trigger the same cascade
How does somatostatin affect ECL cells?
It inhibits them
What is the effect of prostaglandin E2 on acid production?
It acts through a GPCR, but will suppress acid production
Why do drugs, such as aspirin and ibuprofen, increase the risk of gastric ulcers?
These drugs prevent the action of prostaglandin E2
Gastric acid production is not suppressed which increases the risk of gastric ulcers
Why is the secretion of bicarbonate and mucus important?
It protects the epithelium of the stomach from the acidic environment
What is the pH immediately outside of the gastric epithelial cells?
Why?
pH = 7 compared to pH 2 in gastric juice
Goblet cells produce a layer of mucous which sticks close to the epithelial cells
Bicarbonate that is secreted becomes trapped in that mucus layer
How may stomach ulcers be caused by H. pylori infection?
H. pylori resides in the mucus and makes the enzyme urease
Urease breaks down the mucus layer
This allows acid to get through and damage the epithelial cells
What is intrinsic factor and where is it secreted from?
It is a 55 kDa glycoprotein
It is secreted from parietal cells into the stomach
What is the action of intrinsic factor in the stomach?
It is secreted into the lumen with the food but is inactive in the secreted form
It needs to be processed in the small intestine in order to have an effect
What will intrinsic factor bind to in the small intestine?
Why is this important?
It binds to cobalamin (vitamin B12) in the small intestine
This is essential for cobalamin uptake in the ileum
What happens to intrinsic factor in the distal part of the ileum?
There are receptors in the distal part of the ileum that recognise intrinsic factor
They internalise it, and pick up the vitamin B12 in the process
What are chief cells and what do they secrete?
What is the main stimulant of the chief cells?
Chief cells are involved in pepsinogen secretion
There are neurones and interneurons coming into the chief cells, carrying parasympathetic input
How does gastrin have an effect on the chief cells?
Gastrin stimulates the chief cells to release pepsinogen
The G cells detect amino acids, leading to gastrin secretion
Gastrin enters the circulation and exerts an effect on the chief cells
What mediates the effect of gastrin on the chief cells?
How does secretin affect the chief cells?
The effect of gastrin is mediated by CCK-A receptors
Secretin from the duodenum has a much smaller role in pepsinogen secretion
What triggers the cleavage of pepsinogen to pepsin?
What happens to pepsin once it is activated?
Acid secretion from the parietal cells in response to gastrin
Once activated, pepsin is self-perpetuating and can cleave more molecules of pepsinogen
What are pepsins?
What are they secreted in response to?
They are a family of proteases which are secreted from chief and mucus cells
They are secreted in response to acetylcholine and H+
What hormones have minor effects on pepsin secretion?
Secretin, CCK and gastrin
CCK and gastrin work via the CCK-A receptor
How can pepsinogens be cleaved?
They are cleaved spontaneously at low pH (below 5)
Cleavage occurs more quickly is pH is less than 3
Pepsinogens may be cleaved by pepsin - this is autolysis
What is meant by pepsins being endoproteases?
They cut within the peptide chain, rather than at the terminal amino acids
They break the protein into smaller peptides, but not into individual amino acids
What types of proteins to pepsins primarily target?
Amino acids with aromatic or large side chains
They have fairly broad specificity
What is the optimum pH of pepsins?
What pH are they denatured by?
optimum pH is around 2-3
They are denatured when pH is above 5-7, when they reach the small bowel
They are replaced by trypsins coming from the pancreas
What is needed after pepsin to break down the peptides into individual amino acids?
Carboxypeptidases
They are located in the small bowel
Why is ptyalin a-amylase present in the stomach even though it is denatured at pH4?
It is present in the fundus and body where there is lots of food present but it hasn’t mixed with the acid yet
It is NOT produced in the stomach - it comes from salivary glands
How is vomiting controlled?
It is centrally controlled in the area postrema
This is a structure in the medulla oblongata of the brainstem
What 5 things can cause vomiting?
- vagal afferents, in response to irritants in or around the bowel
- psychogenic causes, such as pain and revulsion
- motion sickness/labyrinthine disorders (inner-ear)
- drugs or toxins with a direct effect
- pregnancy
What is the main consequence of vomiting?
Salivation, sweating and hyperventilation
Hypersalivation leads to production of very watery saliva
What is retrograde peristalsis?
The loss of normal gut motility due to vomiting
There are longer peristaltic waves that work backwards from the small bowel, through the stomach and forcing relaxation of the LES
What is retching?
Involuntary contractions of the diaphragm and abdominal wall muscles
What are the other 3 consequences of vomiting?
- displacement of the cardia into the thorax
- reflex closure of the glottis and soft palate and opening of the LES and UES
- emptying of gastric (+/- small bowel) content
